Amplifiers

ABSTRACT

This application relates to amplifier circuits for amplifying an audio signal. An amplifier circuit ( 100 ) has a voltage regulator ( 201 ) for outputting a supply voltage to an amplifier ( 104 ). An output capacitor ( 103 ) coupled to an output node of the voltage regulator. The voltage regulator is operable in a voltage-control mode to maintain the output voltage (V S ) at a nominal output voltage and in current-control mode to limit the input current drawn to exceed a defined limit. A controller ( 301 ) is operable in a first mode to define the nominal output voltage so as not to exceed a first voltage magnitude and in a second mode to define the nominal output voltage to be equal to a second, higher, voltage magnitude. The controller ( 301 ) monitors the audio signal for a high-amplitude part of the audio signal, that could result in the voltage regulator operating in the current-control mode to apply current limiting and, on such detection swaps from the first to the second mode until such a high-amplitude part of the audio signal has been amplified. The second voltage magnitude is greater than required for voltage headroom for amplifying the high-amplitude part of the audio signal so as to allow for a voltage droop of the output voltage over a plurality of switching cycles of the voltage regulator when operating in the current-control mode.

TECHNICAL FIELD

This application relates to amplifier circuits and to methods andapparatus for supplying power to an amplifier circuit, such as a Class-Damplifier circuit, especially for battery powered applications.

BACKGROUND

Many electronic devices have audio amplifiers for generating an audiodriving signal for an output transducer, e.g. a loudspeaker, which mayfor example be an on-board output transducer of the electronic deviceitself.

FIG. 1a illustrates one example of an amplifier circuit 100 as may beimplemented in a battery powered device, for example a portable devicesuch as a mobile telephone or smartphone or tablet computer or the like.A battery 101, in use, provides a power source, at least during some usecases where other sources of power are not available. Power from thebattery 101 may typically, at least in some instances, be regulated by avoltage regulator, such as a DC-DC converter 102 which may, forinstance, be a boost regulator as would be understood by one skilled inthe art. As such a DC-DC converter 102 is a switched power supply, anoutput capacitor 103 is provided to maintain the voltage at the outputof the DC-DC converter 102 across each switching cycle. This voltagemaintained at the output of the DC-DC converter 102 is provided as asupply voltage V_(S) to the amplifier 104.

The amplifier 104 may, in some instances be a Class-D amplifier, asClass-D amplifiers are generally quite power efficient, and powerefficiency is typically an important consideration for portableelectronic devices. However other types of amplifier could be used insome implementations.

The amplifier 104 receives an input signal S_(IN) (which for a Class-Damplifier may be a digital signal) and amplifies the signal to provide adriving signal for driving a load 105, for instance a loudspeaker of thehost device (i.e. the device of which the amplifier circuit 100 forms apart).

As will be understood by one skilled in the art the supply voltage V_(S)provided to the amplifier should be sufficient for the amplifier to beable to amplify the input signal without clipping, allowing headroom foramplifier output stage voltage compliance and DC-DC converter outputvoltage ripple, at least within an expected range of input signalamplitudes.

With a battery powered device however there may be a battery currentlimit specifying the maximum current that should be drawn, in use, fromthe battery 101. Typically therefore the DC-DC converter 102 may have aninput current limit which is set so as to ensure that the input currentto the DC-DC converter 102 does not exceed the battery current limit.

This input current limit effectively limits the power that the DC-DCconverter 102 can output and hence correspondingly the audio power thatcan be output from the amplifier 104.

Increasingly the audio performance specifications for some electronicdevices may be such that a DC-DC converter 102 may be unable to meet thepower demand of the amplifier 104 at peak levels of audio signal S_(IN)without exceeding the battery current limit for practical batteries 101that may be used in such electronic devices. Thus, to avoid unwantedsignal clipping without exceeding the battery current limit may requirethe peak signal level achieved in use to be reduced to below that whichwould be desired.

Embodiments of the present disclosure are thus directed at methods andapparatus for signal amplification that at least mitigate at least someof the above mentioned issues.

Thus according to the present invention there is provided an amplifiercircuit for amplifying an audio signal, comprising:

-   -   a voltage regulator having an input node for receiving an input        voltage and an output node for outputting an output voltage, the        voltage regulator comprising an output capacitor coupled to the        output node;    -   an amplifier connected to receive the output voltage from said        output node of the voltage regulator as a supply voltage;    -   the voltage regulator being operable in a voltage-control mode        to maintain the output voltage at a nominal output voltage and        in current-control mode to apply current limiting so as to limit        an input current drawn by the voltage regulator via the input        node so as not to exceed a defined input current limit; and    -   a controller for defining the nominal output voltage;    -   wherein the controller is operable in a first mode to define the        nominal output voltage so as not to exceed a first voltage        magnitude and is operable in a second mode to define the nominal        output voltage to be equal to a second voltage magnitude, the        second voltage magnitude being greater than the first voltage        magnitude;    -   wherein the controller is configured to monitor an indication of        the level of the audio signal for a high-amplitude part of the        audio signal that could result in the voltage regulator        operating in the current-control mode to apply current limiting;        and    -   wherein the voltage controller is configured to operate in said        first mode until such a high-amplitude part of the audio signal        is detected and, on such detection to operate in the second mode        of operation until such a high-amplitude part of the audio        signal has been amplified;    -   wherein the second voltage magnitude is greater than required        for voltage headroom for amplifying the high-amplitude part of        the audio signal so as to allow for a voltage droop of the        output voltage over a plurality of switching cycles of the        voltage regulator when operating in the current-control mode.

On detection of a high-amplitude part of the audio signal, thecontroller may be configured to change to the second mode of operationsuch that the output capacitor is substantially charged to the secondvoltage magnitude before the high-amplitude part of the audio signal isamplified by the amplifier.

In some embodiments the controller is configured to detect ahigh-amplitude part of the audio signal by determining when theindication of the level of the audio signal exceeds a first thresholdcorresponding to a signal level that would be expected to lead to thevoltage regulator operating in the current-control mode to applyingcurrent limiting. The circuit may comprise a threshold generator fordetermining the first threshold. The threshold generator may beconfigured to determine the first threshold based on the defined inputcurrent limit. In some instances the defined input current limit may beconfigurable and, for instance, may be configured to vary with a batteryvoltage at the input node. The threshold generator may be configured todetermine said the threshold based on at least one of: voltage regulatorefficiency; amplifier efficiency; audio load impedance; audio loadinductance; a minimum audio frequency; and amplifier gain. Thecontroller may comprise a comparator for comparing the indication of thelevel of the audio signal with the first threshold.

In some instance the second voltage magnitude is greater than requiredfor voltage headroom for amplifying the high-amplitude part of the audiosignal so as to allow for a voltage droop of the supply voltage of atleast 10% or at least 15%.

The controller may be configured such that in the first mode ofoperation the nominal output voltage provides a nominal headroom of 15%or less, or 10% or less, of the magnitude of the nominal output voltage.The nominal headroom is the voltage difference between the nominaloutput voltage and the maximum output voltage of the amplifier, in thiscase the maximum output voltage of the amplifier in the first mode ofoperation.

The controller may be configured such that in said second mode ofoperation the nominal output voltage provides a nominal headroom of atleast 20%, or at least 25%, of the magnitude of the nominal outputvoltage. The nominal headroom, in this case, is the voltage differencebetween the nominal output voltage and the maximum output voltage of theamplifier in the second mode of operation.

The controller may be configured to receive a version of the audiosignal and to determine the indication of the level of the audio signal.The amplifier circuit may comprise an envelope detector for receivingthe version of the audio signal and determining an envelope value as theindication of the level of the audio signal.

In some embodiments there is a delay in a signal path for the audiosignal upstream of the amplifier. The delay may comprise a buffer ofconfigurable size. The delay may have a delay duration which is at leastequal to the time that would be required for the output capacitor to becharged to from a lowest expected voltage level in the first mode ofoperation to the second voltage magnitude.

In some implementations, in the first mode of operation, the controlleris configured to vary the nominal output voltage of the voltageregulator based on the indication of the level of the audio signal.

In some implementations, in the first mode of operation, the controllermay be operable to control the voltage regulator in a pass-through modeto pass the input voltage directly to the output node as the supplyvoltage if the indication of the level of the audio signal is below asecond threshold.

The voltage regulator may comprises a DC-DC converter. The controllermay be configured to vary a voltage reference signal supplied to theDC-DC converter to define the nominal output voltage.

The amplifier circuit may be implemented as an integrated circuit. Theamplifier circuit may, in use, comprise a loudspeaker configured to bedriven by the amplifier. In some implementations there may be a batteryand the input node of the voltage regulator may be coupled to receivethe input voltage from said battery. The defined input current limit maycorrespond to a battery current limit.

Aspects relate to electronic device comprising an amplifier circuit asdescribed in any of the variants discussed above. The electronic devicemay be at least one of: a portable device; a battery operated device; acommunication device; a mobile or cellular telephone device; asmartphone; a computing device; a laptop, notebook or tablet computingdevice; a gaming device; a personal media player; a wearable device.

In another aspect there is provided an amplifier circuit for amplifyingan audio signal, comprising:

-   -   an amplifier;    -   a voltage regulator having an input node for receiving an input        voltage and an output node for outputting a supply voltage to        said amplifier, the voltage regulator comprising an output        capacitor coupled to the output node;    -   the voltage regulator comprising a voltage control loop for        controlling the voltage at the output node to a nominal output        voltage and a current control loop for, when activated, limiting        an input current drawn by the voltage regulator via the input        node so as not to exceed a defined input current limit, wherein        the voltage regulator operates according to the voltage control        loop unless the current control loop is activated; and a        controller for defining the nominal output voltage;    -   wherein the controller is operable in a first mode to define the        nominal output voltage so as not to exceed a first voltage        magnitude and is operable in a second mode to define the nominal        output voltage to be equal to a second voltage magnitude, the        second voltage magnitude being greater than the first voltage        magnitude;    -   wherein the controller is configured to monitor an indication of        the level of the audio signal for a high-amplitude part of the        audio signal that could result in the current control loop of        the voltage regulator being activated; and    -   wherein the controller is configured to operate in said first        mode until such a high-amplitude part of the audio signal is        detected and, on such detection to operate in the second mode of        operation until such a high-amplitude part of the audio signal        has been amplified;    -   wherein the second voltage magnitude is greater than required        for voltage headroom for amplifying the high-amplitude part of        the audio signal so as to allow for a voltage droop of the        supply voltage over a plurality of switching cycles of the        current control loop dominating the voltage control loop.

In a further aspect there is an amplifier circuit for amplifying anaudio signal, comprising:

-   -   an amplifier;    -   a voltage regulator having an input node for receiving an input        voltage and an output node for outputting a supply voltage to        said amplifier, the voltage regulator comprising an output        capacitor coupled to the output node;    -   the voltage regulator being operable to apply current limiting        so as to limit an input current drawn by the voltage regulator        via the input node so as not to exceed a defined input current        limit;    -   a controller for defining a nominal output voltage generated by        the voltage regulator;    -   wherein the controller is operable to compare an indication of        the level of the audio signal to a first threshold, wherein the        first threshold is indicative of the input signal that, if        amplified by the amplifier, would be expected to lead to the        voltage regulator experiencing current limiting; and    -   in the event that the indication of the level of the audio        signal exceeds said first threshold to increase the magnitude of        the nominal output voltage of the voltage regulator from a first        operating range to a second higher voltage level.

A yet further aspect provides an amplifier circuit for amplifying anaudio signal, comprising:

-   -   an amplifier;    -   a voltage regulator having an input node for receiving an input        voltage and an output node for outputting a supply voltage to        said amplifier, the voltage regulator comprising an output        capacitor coupled to the output node;    -   the voltage regulator being operable to apply current limiting        so as to limit an input current drawn by the voltage regulator        via the input node so as not to exceed a defined input current        limit;    -   a threshold generator for generating a first threshold        indicative of an audio signal level that would be expected to        lead to current limiting being applied by the voltage regulator;        and    -   a controller for controllably varying a nominal output voltage        generated by the voltage regulator;    -   wherein the controller is operable in a first mode to control        the nominal output voltage so as not to exceed a first voltage        magnitude and in a second mode to control the nominal output        voltage to be equal to a second voltage magnitude, the second        voltage magnitude being greater than the first voltage        magnitude;    -   wherein the controller operates in the first mode unless and        until the indication of the level of the audio signal exceeds        the first threshold and then changes to the second mode of        operation until a predetermined time after the level of the        audio signal has dropped back below the first threshold.

In another aspect there is an amplifier circuit for amplifying an audiosignal, comprising:

-   -   an amplifier;    -   a voltage regulator having an input node for receiving an input        voltage and an output node for outputting a supply voltage to        said amplifier, the voltage regulator comprising an output        capacitor coupled to the output node;    -   the voltage regulator being operable to apply current limiting        so as to limit an input current drawn by the voltage regulator        via the input node so as not to exceed a defined input current        limit; and    -   a controller for controlling a nominal output voltage generated        by the voltage regulator;        wherein the controller is operable in a first mode to control        the nominal output voltage so as not to exceed a first voltage        magnitude and is operable in a second mode to control the        nominal output voltage to be equal to a second voltage        magnitude, the second voltage magnitude being greater than the        first voltage magnitude;    -   wherein the controller is configured to monitor an indication of        the level of the audio signal for a high-amplitude part of the        audio signal that could result in the voltage regulator applying        current limiting; and    -   wherein the controller is configured to operate in said first        mode until such a high-amplitude part of the audio signal is        detected and, on such detection to operate in the second mode of        operation until such a high-amplitude part of the audio signal        has been amplified;    -   wherein the second voltage magnitude is greater than required        for standard voltage headroom for amplifying the high-amplitude        part of the audio signal.

In another aspect there is an amplifier circuit for amplifying an audiosignal, comprising:

-   -   an amplifier;    -   a voltage regulator having an input node for receiving an input        voltage and an output node for outputting a supply voltage to        said amplifier, the voltage regulator comprising an output        capacitor coupled to the output node;    -   the voltage regulator being operable to apply current limiting        so as to limit an input current drawn by the voltage regulator        via the input node so as not to exceed a defined input current        limit; and    -   a controller for controlling a nominal output voltage generated        by the voltage regulator;    -   wherein the controller is configured to monitor an indication of        the level of the audio signal for a high-amplitude part of the        audio signal above a first threshold; and    -   wherein the controller is configured to operate in a first mode        until such a high-amplitude part of the audio signal is detected        and, on such detection to operate in the second mode of        operation until such a high-amplitude part of the audio signal        has been amplified;    -   wherein in the first mode of operation the nominal output        voltage provides a nominal voltage headroom of 10% or less of        the magnitude of the nominal output voltage; and    -   wherein in the second mode of operation the nominal output        voltage provides a nominal voltage headroom of at least 25% of        the magnitude of the nominal output voltage.

BRIEF DESCRIPTION OF DRAWINGS

To explain various aspects of the present disclosure various embodimentswill now be described, by way of example only, with reference to theaccompanying drawings, of which:

FIG. 1a illustrates a conventional amplifier arrangement and FIGS. 1band 1c illustrate conventional operation of a DC-DC converter that maybe used to supply an amplifier;

FIGS. 2a and 2b illustrates example voltage waveforms illustratingrespectively the concepts of voltage ripple and headroom and using ahigh fixed supply voltage for an amplifier;

FIG. 3 illustrates an amplifier arrangement according to an embodiment;

FIG. 4 illustrates example voltage waveforms for the embodiment of FIG.3;

FIG. 5 illustrates one example of determining a voltage threshold;

FIG. 6 illustrates an amplifier arrangement according to anotherembodiment; and

FIG. 7 illustrates example voltage waveforms for the embodiment of FIG.6.

DETAILED DESCRIPTION

Referring to FIG. 1a , as discussed above, the audio performancerequirements for some battery powered electronic devices, e.g. portableelectronic devices such as tablet computers or mobile phones, may besuch that increasingly that a voltage regulator, e.g. comprising DC-DCconverter 102, which regulates power supply to the amplifier 104, maynot be able to supply sufficient power at peak signal levels of inputsignal S_(IN) without exceeding a battery current limit for current drawfrom the battery 101. In essence an input current limit I_(lim) for theDC-DC converter 102, which may be set based on a specified batterycurrent limit value, results in a limit in the output current that theDC-DC converter can supply. Consequently there is a maximum outputvoltage level that be driven from the amplifier to a particular load,given the characteristics of the load, without requiring more outputcurrent from the DC-DC converter 102 than it can supply.

FIG. 1b illustrates an example DC-DC converter 102, in this case a boostconverter, which provides an output supply current I_(S) at a converteroutput node, in turn pulling a current I_(batt) from a battery whichprovides a battery voltage V_(batt). This battery voltage may beprovided either directly from the battery 101 or via some decouplingnetwork (not illustrated). The battery voltage is applied to an inductor106 which is also connected to switches 160 and 161. Control terminalsof switches 160 and 161 are connected to switch controller 150 which mayreceive a clock signal CLK.

The switch controller 150 controls the operation of switches 160 and161. With switch 160 active, i.e. on, and switch 161 off, the inductoris connected between the battery voltage and a reference, typicallyground. This allows the current in the inductor 106 to build up. Withswitch 160 off and switch 161 on or active the inductor is insteadconnected to the output node and the current stored in the inductorpasses to the output terminal and recharges the output capacitor 103.The switch controller 150 thus controls the switches 160 and 161 tooperate the DC-DC converter in at least two switch states: a firstswitch state with switch 160 off and switch 161 on where stored currentis supplied from the inductor to the output; and a second switch statewith switch 160 on and switch 161 off where current is being stored inthe inductor.

The output voltage V_(S) is fed-back to a first error amplifier 151 andcompared to a nominal output voltage V_(NOM) and a resultant voltageerror signal is provided to the controller 150 which consequentlyadjusts the timing of pulses to the control terminals of the switches160 and 161 so as to close a first, voltage-control, feedback loop toregulate the output voltage V_(S) to be close to V_(NOM). In someinstances the controller 150 may switch to a defined one of the first orsecond switch states based on the clock signal and then swap to theother switch state based on the feedback error signal. A switching cycleis thus defined by the clock signal with the duty cycle of a particularswitch state within the switching cycle being determined by the errorfeedback loop.

Many circuits for providing this feedback loop are known. It will benoted that FIG. 1b illustrates the output voltage V_(S) being comparedto the desired nominal voltage but it will be understood that a levelshifted version of the output voltage could be compared to a suitablereference. In general though the voltage control loop including firsterror amplifier 151 operates to maintain the output voltage, as far aspossible, at the level of the nominal output voltage.

If the load current increases, then the current I_(batt) taken from thebattery will also increase as illustrated in FIG. 1c . At some point thecurrent draw may increase to a value I_(lim) that is the maximumspecified current that is allowed to be taken from the battery by theDC-DC converter (possibly taking into account other anticipated loadingof the battery).

To provide current limiting the battery current drawn may be monitoredand compared to I_(lim) by a second error amplifier 152. If the drawnbattery current I_(batt) is less than I_(lim) then the output of erroramplifier 152 has no effect on the pulse outputs of switch controller150, i.e. the timing or duty-cycles of the switch states. However ifI_(batt) attempts to exceed I_(lim) then a current-control loop viaerror amplifier 152 is arranged to dominate the voltage-control feedbackloop and to keep I_(batt) close to I_(lim) despite any resultingdecrease in the output voltage V_(S).

Thus the DC-DC converter 102 can, in general, operate in either avoltage-control mode or a current-control mode i.e. an activecurrent-limiting mode, depending on the current demanded from thebattery and thus indirectly on the current required to supply the load.In the voltage-control mode the current control loop, i.e. erroramplifier 152, has no substantially active effect on the switch statesand the output voltage generated at the output node of the voltageregulator is controlled to be substantially equal to the defined nominalvoltage V_(NOM). In the current-control mode however the switching ofthe switch states may be modified by the current control loop to avoidexceeding the input current limit.

When used to provide a supply voltage to an amplifier 104, asillustrated in FIG. 1a , it will be understood that as the signalvoltage to be applied to a given load increases the current requiredincreases. Consequently, there is a maximum output voltage level thatcan be driven from the amplifier 104 to a particular load, given thecharacteristics of the load, without requiring more output current fromthe DC-DC converter 102 than it can supply due to the constraint on itsinput current.

One way to overcome this issue would be to use a dedicated energystorage element, such as a capacitor, to store energy to support theload demand in such circumstances. The energy storage element could becoupled to the voltage supply rail for the amplifier 104. In the eventthat the DC-DC converter 102 can meet the load demand the energy storageelement would remain charged. However if the DC-DC converter entered anactive current limiting mode of operation, i.e. the input current limitfor the DC-DC converter 102 was reached and the DC-DC converter wasunable to meet the load demand, the energy storage element could supplyadditional charge to effectively make up the shortfall.

The energy storage element could be a capacitor in addition to theoutput capacitor 103. However this would require the presence of anadditional capacitor, which would add to the cost and size of thecircuitry and may require additional integrated circuit contactterminals, e.g. pins. For portable electronic devices size of thevarious components and the resulting PCB area occupied can be anotherimportant consideration.

The amplifier circuit 100 could therefore be operated to use the outputcapacitor 103 as an energy storage capacitor which is sufficient tostore energy to support the load when the DC-DC converter 102experiences current limiting. However, to provide a useful amount ofenergy storage, the capacitance of the output capacitor 103 would needto be relatively large, for instance significantly larger thanconventionally may be used just to maintain the output voltage of theDC-DC converter 102 across each cycle of the various switch states ofthe DC-DC converter.

As discussed with reference to FIG. 1b a DC-DC converter, such as aboost converter or the like, typically switches between at least twoswitch states in a cycle to provide the required output voltage. In afirst state of the DC-DC converter, a voltage is developed at the outputof the DC-DC converter to meet the load demand and (in normal operation)charge the output capacitor. In a second state the DC-DC converter iseffectively charging/discharging other components such as an inductorand the output capacitor at least partially supports the load demand. Innormal, non-current limited operation, i.e. under voltage-control, theswitching cycle is controlled, in line with the loading on the DC-DCconverter, to provide the desired nominal output voltage. The outputcapacitor 103 is typically chosen to be just large enough to support theload demand between instances of the first state, i.e. between instancesof the output capacitor being re-charged, e.g. during instances of thesecond state, with an acceptable voltage ripple for the highest expectedload demand, but assuming that, on average, the DC-DC converter can meetthe power demand. The output capacitor 103 would thus not normally belarge enough to supply charge over a prolonged period of time,comprising many switching cycles, where the DC-DC converter itself wasunable to meet the power demand without an unacceptable voltage droop.

Thus, as mentioned, one could consider using an output capacitor that issignificantly larger than one that would conventionally be used in orderto store additional charge. However using a significantly largercapacitance for the output capacitor 103 would itself result inincreased size and cost and may be undesirable.

Instead therefore the voltage output from the DC-DC converter 102, i.e.the nominal output voltage V_(S), may be significantly increasedcompared to what otherwise might be required for the amplifier 104.

It will be understood that conventionally the DC-DC converter 102 may beoperated to provide a supply voltage V_(S) which has sufficient voltageheadroom for the amplifier 104 to amplify to a desired peak signalvoltage given the normal expected voltage ripple.

FIG. 2a illustrates the principles of headroom and ripple voltage. FIG.2a illustrates the supply voltage V_(S) and a plot 201 of output voltagefrom the amplifier 104. As mentioned above the DC-DC converter 102 willgenerally be a switched power supply that repeatedly cycles through asequence of switch states including a first state in which the nominaloutput voltage V_(NOM) is developed at the output of the DC-DC converter102 and a second state in which the output capacitor at provides atleast some of the output load current load to maintain the supplyvoltage. Thus there will be repeating cycle including periods P1 of thefirst state and periods P2 of the second state. During a period P2 ofthe second state the output capacitor 103 is supporting the load andbeing discharged, there is thus a slight droop in the supply voltageV_(S) over this period P2 that depends on the loading. During a periodP1 of the first switch state the voltage at the output of the DC-DCconverter 102 recharges the output capacitor. This results, over theswitching cycle, in a voltage ripple of magnitude ΔV_(R). The switchingfrequency and/or duty cycle of the DC-DC converter 102, i.e. durationsof periods P1 and P2, is controlled so that the output capacitor isrecharged, as far as possible, to V_(NOM) each cycle and the magnitudeof the voltage ripple ΔV_(R) at the worst case loading is withinacceptable limits. Note that the magnitude of the voltage droop and theduration of the switching period with respect to variations in theoutput signal (which is an amplified version of the input signal S_(IN))are exaggerated in FIG. 2a in order to explain the principles.

For amplifier 104 to correctly amplify the input signal S_(IN) withoutclipping, the supply voltage V_(S) to the amplifier should allow for acertain amplifier headroom H_(A) on top of the amplified output signal201. This amplifier headroom H_(A) is effectively the minimum additionalvoltage required to allow correct operation of the amplifier, forinstance to allow for the voltage drop arising from the on-resistance ofthe output transistors of the amplifier. The amount of headroom requiredfor a given amplifier will depend on the amplifier design and may alsodepend on signal level and also on operating conditions. The amplifierheadroom H_(A) required may therefore be defined for an amplifier basedon the minimum headroom required, under worst case expected operatingconditions, at the maximum expected output signal level V_(SMAX). Toallow for manufacturing tolerances of various circuit components anduncertainty in the operating conditions, a certain tolerance may beincluded in the amplifier headroom H_(A), however this is usuallyrelatively small as it is generally preferred to keep the nominal supplyvoltage as low as practical. One skilled in the art would readilyunderstand how to determine an appropriate amplifier headroom H_(A) fora particular amplifier implementation.

The voltage ripple ΔV_(R) together with the amplifier headroom H_(A) canbe seen as collectively defining an operating headroom H_(O) which isrequired for correct amplification of the signal. As used herein theterm operating headroom H_(O) will thus be used to refer to the minimumheadroom required between the nominal output voltage V_(NOM) and themaximum output signal level V_(SMAX) for correct operation, based on theworst case expected operating conditions (H_(O)=ΔV_(R)+H_(A)).

The nominal output voltage V_(NOM) of the DC-DC converter may thereforebe set based on the determined operating headroom H_(O) required for therelevant circuit and the maximum expected output signal level V_(SMAX).

In some embodiments the maximum expected output signal level V_(SMAX)may be limited by physical constraints of the audio signal path, inother words by limiting the maximum allowed level of the input signalS_(IN). The range of the input signal S_(IN) may be limited, forinstance, by the output range of components upstream of amplifier 104 inthe signal path. For example a maximum input signal level may bedetermined, at least partly, by the highest gain setting of a variablegain element that provides some volume control and/or a full-scaleoutput voltage and/or maximum modulation factor of a DAC in the signalpath. In some embodiments however a maximum input signal level may be anotional design limit; that is the amplifier circuit will be designed tobe able to amplify any signal up to a maximum input level withoutclipping but may physically, in some instances, be able to be operatedto supply a higher signal level to the amplifier, albeit with a risk orlikelihood of signal clipping.

The term nominal headroom H_(NOM) will be used herein to refer to theactual difference between the nominal output voltage V_(NOM), i.e. thenominal supply voltage, and the maximum expected output signal levelV_(SMAX) (H_(NOM)=V_(NOM)−V_(SMAX)). The nominal headroom H_(NOM) for anamplifier circuit should thus be set to be at least equal to therequired operating headroom H_(O). The nominal headroom can thus be seenas an indication of the minimum actual headroom that may be expected inuse (if the supply voltage were at the nominal output voltage level andthe output signal were at the expected maximum level V_(SMAX)). In theexample illustrated in FIG. 2a the nominal headroom H_(NOM) is arrangedto be equal to the operating headroom H_(O) required.

It will also be appreciated that the actual voltage difference betweenthe output signal and the supply voltage will vary over time as thesignal level changes and if the output signal level at a given point intime is lower than the expected maximum level V_(SMAX), e.g. asrepresented by the dotted line 201 _(B), the actual voltage differencebetween the output signal 201 at that time and the supply voltage V_(S)may be much greater than required (but the nominal headroom is definedwith regard to the maximum signal level and does not vary).

In some amplifier designs, as will be discussed in more detail below,the value of the nominal output voltage V_(NOM) of the DC-DC converter102, i.e. the nominal supply voltage, may be varied in use to reducewhen the signal level reduces so as to reduce any excess actualheadroom. If the nominal output voltage V_(NOM) is arranged to vary withsignal level, for example in Class-G or Class-H modes of operation, therelevant maximum expected output signal level at any given time will bethe maximum output signal level at which that respective nominal supplyvoltage at that time is used. In other words in a system where thenominal supply voltage changes with signal level there will be a nominalheadroom associated with each nominal output voltage based on themaximum signal level that corresponds to that output voltage.

As an example in some implementations a battery voltage of around 3V maybe boosted to a supply voltage V_(S) of around 8V to be suitable for aClass-D amplifier driving an on-board loudspeaker. In normal operationthe magnitude of the voltage ripple ΔV_(R) may be of the order of 100 mVor so in some implementations. The required amplifier headroom may be ofthe order of a few hundred millivolts, say around 200 mV in someimplementations to allow for the on-resistance of output transistors ofthe size that may be typically used and also some possible constraint onthe maximum and minimum duty cycle of the periods P1/P2. The operatingheadroom H_(O) may therefore be of the order of a few hundred my. Thenominal headroom for such an amplifier may therefore be set to be of theorder of 500 mV or so to cope with this expected voltage ripple andamplifier voltage drop. The maximum output signal level, when using the8V supply voltage, may therefore be limited to around 7.5V. This may beseen as standard headroom, where the nominal headroom is of the samesort of magnitude as the required operating headroom, i.e. any excessheadroom at the maximum signal level is relatively small.

In order to provide reasonable energy storage in the output capacitor103, without requiring an unduly large capacitance, the DC-DC converterinstead may be operated with a significantly higher output voltage, sayof the order of 12V. The supply voltage could therefore be set at alevel which is significantly greater than required for voltage rippleand amplifier headroom H_(A) reasons.

In use, the DC-DC converter 102 would supply power to the amplifier 104as described previously but in this case the supply voltage would be afixed relatively high voltage supply, say a supply of 12V. The inputcurrent limit for the DC-DC converter 102 would be set to ensure thatthe current draw from the battery does not exceed the battery currentlimit. In at least some operating conditions, e.g. at relatively lowsignal levels, the power draw by amplifier 104 may be such that thecurrent demand can be met by DC-DC converter 102 without exceeding theinput current limit. If, however, the operating conditions change, e.g.the audio signal level approaches a peak signal level, the DC-DCconverter 102 may enter a current limited mode of operation where theinput current to the DC-DC converter is actively being limited. In thiscase the output current from the DC-DC converter 102 may be insufficientto meet the demand from the amplifier 104, however the extra energystored in the output capacitor will support the load demand.

FIG. 2b illustrates the principles of such operation. FIG. 2b indicatesa plot 201 representing the output signal from amplifier 104 over time.Also shown is the supply voltage V_(S) and how it varies over time. Thelower plot of FIG. 2b represents an indication of when current limitingis active in the DC-DC converter 102, e.g. the DC-DC converter hasswapped from a normal voltage mode of control to a current-mode control.Note it will be understood that a current limit monitor (e.g. the seconderror amplifier 152 of FIG. 1b ) will normally operate continuously whenthe DC-DC converter is operating to ensure that the relevant inputcurrent limit is not exceeded. Thus current limiting control ormonitoring may always be operating, however as used herein the termcurrent limiting or a current limited mode of operation shall be used tomean that input current is actively being limited, i.e. reduced comparedto what the input current would be in the absence of the currentlimiting control. Current limiting can be said to be active if thepresent input current depends, at that time, on the input current limitdefined for the DC-DC converter 102. For the example DC-DC converterillustrated in FIG. 1b current limiting can be said to be active if theswitch controller 150 is responsive to the output of the current controlloop, i.e. the output of second error amplifier 152, to override thetiming indicated by the voltage error feedback loop, i.e. output offirst error amplifier 151.

At the start of the time period illustrated by FIG. 2b the supplyvoltage V_(S) is at a high fixed level, say 12V. Note that FIG. 2b isconcerned with changes on the timescale of the audio input signal S_(IN)and not on the much faster switching frequency of the DC-DC converterand ignores any ripple voltage associated with the switching phases ofthe DC-DC converter 102 for clarity, however an indication of how theactual voltage may vary is illustrated in the expanded section 202).Initially, up to a time t1, the power demand of the amplifier 104 can bemet by the DC-DC converter 102 without exceeding the input currentlimit. At time t1 however the signal level of the amplified outputsignal 201 has reached a level where the DC-DC converter 102 reaches theinput current limit. After this time, until time t2, the DC-DC converteris experiencing current limiting and the DC-DC converter 102 cannot meetthe full current demand. However the output capacitor 103 can supplycharge to ensure that the full current demand is met. As such thevoltage of the supply voltage V_(S) as maintained by the outputcapacitor 103 may begin to droop (over several switching cycles of theDC-DC converter). However as the initial supply voltage wassignificantly higher than required by the amplifier 104 the amplifier104 still continues to operate correctly. At time t2 the signal levelreduces to a level where the DC-DC converter 102 can meet the powerdemand without current limiting. The output capacitor is thus rechargedback up to the high output voltage of the DC-DC converter 102, e.g. 12V,and is thus ready support load demand for another subsequent signalpeak.

Whilst such an approach may provide the ability to meet the peak audiodemand whilst not exceeding a battery current limit, it does howeverrequire a relatively high supply voltage to be maintained, which is notdesirable for power efficiency. As noted above the DC-DC converter 102should be operated to provide a supply voltage V_(S) which hassufficient voltage headroom for the signal being amplified. However ifthe difference between the signal being amplified and the supply voltageV_(S) is greater than a required voltage headroom, i.e. there is excessheadroom, then there may be unnecessary power wastage in the amplifier.Also the DC-DC converter may consume more power when operating with ahigher output voltage.

Operating with a higher supply voltage than is necessary for standardheadroom reasons thus tends to reduce power efficiency as inherentlymore power is wasted in amplifier operation and the DC-DC converter.

Embodiments of the present disclosure relate to an amplifier arrangementin which an amplifier receives power from a voltage regulator and acontroller is configured to receive an indication of the level of asignal to be amplified by the amplifier and to determine whether thesignal to be amplified is likely to result in the voltage regulatorexperiencing current limiting. In the event that it is determined thatcurrent limiting being applied at the voltage regulator is likely, e.g.based on known or assumed system parameters, and thus the voltageregulator may be unable to meet the power demand from the amplifier,then the amplifier circuit may operate with the voltage regulatoroutputting a defined high output voltage. This defined high voltage maybe a relatively high voltage, which is higher than required just forstandard voltage headroom reasons, in order to store energy in acapacitor or other energy storage element coupled to the supply voltagerail in a similar manner as described with reference to FIG. 2b .However when it is determined that voltage regulator is unlikely toenter a current limiting mode of operation, and the power demand of theamplifier may be met by the voltage regulator itself, there may be noneed to store such additional energy in an output capacitor or similar.At such times therefore the voltage output from the voltage regulatormay be reduced from the defined high voltage, and possibly significantlyreduced, and may be set based on the expected signal level and standardheadroom. The voltage regulator may comprise a DC-DC converter.

Thus for example, as mentioned above a supply voltage of 8V may be asuitable for a Class-D amplifier for driving a loudspeaker when theDC-DC converter is not operating in a current limited mode of operation,e.g. for a first range of signal amplitudes. If the controllerdetermines that the DC-DC converter will not operate in a currentlimited mode of operation the DC-DC converter may be operated to providean output voltage of 8V. If however it is determined that a high levelpart of the input signal will likely cause the power demand from theamplifier to result in the DC-DC converter experiencing currentlimiting, then the output voltage from the DC-DC converter may beincreased to the defined high voltage, say 12V, to store energy in theoutput capacitor. This defined high voltage may be significantly higherthan required for standard headroom reasons. The output voltage isincreased to the defined high voltage in sufficient time before the highsignal level part of the signal reaches the amplifier, such that thereis time to charge the output capacitor, or other storage element, to thedefined high voltage to store charge to support the power demand (forwhen the DC-DC converter does enter current limiting). Thus thecontroller may be part of, or responsive to, a look ahead module thatanalyses an indication of signal level sufficiently far ahead of thesignal being amplified.

FIG. 3 illustrates one embodiment in which similar components areidentified by the same reference numerals. In the embodiment of FIG. 3the input signal S_(IN) is tapped from a main signal path upstream ofthe amplifier 104 and passed to controller 301. The controller 301analyses the input signal S_(IN) to determine whether that part of thesignal, when amplified, will create a power demand that will likely leadto current limiting be applied within DC-DC converter 102. In someinstances the controller may determine a high-amplitude part of theinput signal S_(IN) by determining whether the input signal level isabove a predetermined threshold.

The controller 301 selectively controls the output voltage of thevoltage regulator, i.e. DC-DC converter 102. The controller mayselectively operate in one of at least two modes. In the event that nohigh-level part of the input signal S_(IN) is detected the controller301 may operate in a first mode of operation and the output voltage fromthe DC-DC converter 102 may be controlled to be at, or no greater than,a first voltage V1. In this first mode of operation the supply voltagemay be set to provide a standard amount of voltage headroom, i.e. thenominal headroom H_(NOM) may be of the order of the required operationheadroom H_(O). In the event however that it is detected that a part ofthe input signal S_(IN) will likely lead to current limiting beingapplied within the DC-DC converter 102, then the controller 301 operatesin a second mode of operation and controls the DC-DC converter 102 suchthat the nominal output voltage from the DC-DC converter 102 is a secondvoltage, V2, which is higher (and possibly significantly higher) thanthe first voltage V1. The controller 301 switches to the second mode ofoperation and increases the output voltage of the DC-DC converter 102before the high-amplitude part of the signal reaches the amplifier 104and in sufficient time that the output capacitor 103 can be charged to avoltage greater than the first voltage V1, for example in time thatoutput capacitor can be charged to substantially the second voltage V2.The controller 301 thus switches to the second mode of operation beforethe DC-Dc converter enters an active current limiting mode of operation,and sufficiently before that the output capacitor 103 can be charged tosubstantially the second voltage V2. In this second mode of operationthe supply voltage may be set significantly higher than is required forstandard headroom, i.e. the nominal headroom H_(NOM) is significantlyhigher than the required operational headroom H_(O). Thus this secondmode of operation may deliberately provide a significant excess headroom(H_(E)=H_(NOM)−H_(O)) in order to store sufficient energy in the outputcapacitor. In effect the excess headroom is provided to allow asignificant voltage droop of the supply voltage V_(S) over a pluralityof cycles of the DC-DC converter 102 whilst still maintain the requiredoperating headroom H_(O).

In order to provide this look-ahead functionality the controller 301taps the input signal S_(IN) from the signal path ahead of some delaybefore the amplifier 104. The delay may at least partly implemented bysome deliberate delay element 302, e.g. a suitable buffer, to give asufficient delay D between the controller identifying a high-level partof the signal and the high-level part of the signal reaching theamplifier 104. However in some instances at least part of the delay maybe due to inherent propagation delays in the downstream signal pathand/or other signal processing applied to the input signal S_(IN) beforeamplification (although if such downstream signal processing varies thesignal level being amplified the controller 301 should be configured totake such downstream processing into account). The controller 301 anddelay element 302 can collectively be seen as being at least part of alook-ahead module 303.

FIG. 4 illustrates voltage waveforms similar to those illustrated in theFIG. 2b to show one example of operation according to the embodiment ofFIG. 3. FIG. 4 illustrates the same amplified signal 201 as shown inFIG. 2b . Again this signal 201 will result in the DC-DC converter 102experiencing current limiting between times t1 and t2. In this instancehowever, as initially the level of the signal being amplified will notresult in current limiting of the DC-DC converter 102, the outputvoltage from the DC-DC converter 102 is initially set at the firstvoltage V1 and thus the supply voltage V_(S) to the amplifier 104 isalso at V1.

The controller 301 effectively analyses the input signal S_(IN) ahead ofthe signal being amplified by a period equal to the delay D. Thecontroller may, in one embodiment, detect a high-amplitude part of thethreshold by detecting when the input signal exceeds a thresholdV_(THR). For the purposes of explanation FIG. 4 illustrates a thresholdV_(THR)′ which is scaled appropriately for the amplified signal, but itwill be appreciated that this may simply be a scaled version of athreshold V_(THR) which may applied to the input signal S_(IN). At atime t0 the controller 301 detects a high-amplitude part of the inputsignal that will eventually appear at the output at time t1. Thecontroller 301 thus switches to the second mode of operation at thistime, i.e. t0, and increases the nominal output voltage of the DC-DCconverter 102 to V2. The output capacitor 103 is thus charged to thevoltage V2 over a period before the high level part of the input signalis amplified at time t1. When the high level part of the input signal isamplified at time t1, and the DC-DC converter 102 then experiencescurrent limiting, the energy stored in the output capacitor 103 helpssupport the power supply to the amplifier 104 which thus amplifies thesignal without clipping. As the power demand of the amplifier can nolonger be met by the DC-DC converter 102 the charge of the outputcapacitor 103 is depleted over time (and over a period that mayencompass several tens or hundreds of switching cycles of the DC-DCconverter) and the supply voltage V_(S) droops, but remains sufficientto provide sufficient voltage headroom for the amplifier 104, i.e. thesupply voltage remains above the signal level plus the requiredoperational headroom H_(O). In other words the excess headroom providedby the high voltage V2 is sufficient to allow a significant voltagedroop of the supply voltage V_(S) and still provide the requiredoperating headroom H_(O).

At time t2 the signal 201 being amplified has again dropped to a lowlevel and thus the controller may be changed back to the first mode ofoperation with a nominal output voltage of V1 from DC-DC converter 102.The output capacitor 103 will thus discharge back down to V1. Thecontroller 301 may thus increase the output voltage of DC-DC converter102 to the higher voltage V2 when a high-amplitude part of the inputsignal is detected, e.g. the input signal level is above the threshold.This high voltage operation may be maintained whilst the input signallevel remains above the threshold and for a period of time afterwardscorresponding to the delay D so that all of the high amplitude part ofthe signal is amplified before changing back to the first mode ofoperation at the lower output voltage.

As mentioned previously, in the first mode of operation the supplyvoltage V1 may be set to provide standard headroom. The maximum outputsignal level (V_(SMAX)) for the first mode of operation is effectivelyequal to the scaled threshold V_(THR)′—as an input signal above V_(TH)would result in swapping to the second mode of operation. The supplyvoltage V1 may therefore be set to provide standard headroom for amaximum output signal level of the scaled threshold V_(THR)′. Forexample, consider that the threshold V_(THR) corresponds to an outputsignal level of 7.5V. A nominal headroom H_(NOM) of about 500 mV may besuitable for standard headroom (to provide sufficient amplifier headroomH_(A) and to allow for voltage ripple). Thus the voltage V1 may be setto be equal to 8V. In this example the nominal headroom is thus of theorder of just over 6% of the supply voltage.

In various embodiments in the first mode of operation the magnitude ofthe nominal headroom in the first mode may be of the order of 15% orless of the magnitude of the nominal supply voltage, or of the order of10% or less. This may be seen as a standard headroom.

In some embodiments the magnitude of the nominal headroom may be of theorder of 1V or less.

In the second mode of operation the supply voltage is set so as to storesufficient energy in the output capacitor, or some other energy storageelement coupled to the supply rail. In this case the nominal supplyvoltage is set to provide significant excess headroom so thatsignificant droop of the supply voltage below the nominal voltage can beexperienced, over many switching cycles, and the actually supply voltageremains high enough to provide the operating headroom H_(O). Thecontroller 301 swaps to the second mode of operation when the inputsignal S_(IN) exceeds the threshold V_(THR) which, in the example above,corresponded to an output signal level of 7.5V. In the second mode ofoperation the maximum expected output signal level may be limited bydesign as discussed above, and may, for example, be of the order of8.5V. In the second mode of operation the supply voltage may be raisedto a nominal supply voltage of 12V say, i.e. V2=12V in this example. Thenominal headroom in the second mode of operation is thus 3.5V, which issignificantly higher than the 0.5V that may be required for standardheadroom reasons. The nominal headroom in this case is of the order of30% of the magnitude of the nominal supply voltage.

In some embodiments therefore the magnitude of the nominal headroom inthe second mode of operation may be of the order of at least 20% of themagnitude of the nominal supply voltage, or at least 25% of themagnitude of the nominal supply voltage.

In some embodiments the excess headroom (i.e. H_(NOM)−H_(O)) in thesecond mode may be of the order of at least 10% of the magnitude of thenominal output voltage. In the first mode any excess headroom may belower than 10% of the magnitude of the nominal output voltage and may belower than 5% of the magnitude of the nominal output voltage. In otherwords the excess headroom in the second mode of operation may besufficient to allow a voltage droop of at least 5% of the supply voltagefrom the nominal value, or a voltage droop of at least 10%, or in someembodiments at least 15% or at least 20%. Such a large voltage droopcould not be tolerated if standard voltage headroom were to be provided.

In some embodiments the nominal headroom in the second mode may be ofthe order of at least 1V or at least 2V.

It will of course be appreciated that any specific figures for supplyvoltage and headroom are given by way of example only and are notlimiting. The exact figures will depend on a particular implementation.

In this way peaks of the signal are supported by the output capacitor103 being pre-charged to a high level but only when required, thusenabling the amplifier to be operated with a lower voltage supplyvoltage at other times when the high voltage is not required, thusgreatly increasing power efficiency.

It will be appreciated by one skilled in the art that there is somesimilarity to Class-G or Class-H amplifier techniques. In a Class-G orClass-H amplifier the supply voltage V_(S) supplied to the amplifier isvaried based on an indication of the signal to be amplified. If thesignal S_(IN) to be amplified has a relatively low level the supplyvoltage V_(S) may be reduced to reduce the excess voltage headroom andconsequently reduce power wastage. If the input signal level S_(IN) thenincreases the supply voltage may be increased to provide appropriatevoltage headroom. There may be relatively continuous tracking of thesupply voltage V_(S) with the signal level or the supply voltage may bevaried between one or more set levels when the signal level falls invarious bands, e.g. by comparison with one or more thresholds.

Conventional class-G and class-H operation however is purely concernedwith voltage headroom and adjusts the voltage purely to ensure there issufficient voltage headroom for the present signal voltage or signalrange. At each operating voltage it is expected that the DC-DC converter102 can meet the power demand of the amplifier and any output capacitor103 is only charged to a level to support the load demand between thecharging phases of the DC-DC converter 102. Thus for class-G or class-Hoperation the supply voltage will be defined based on the requiredstandard operating headroom, i.e. amplifier headroom H_(A) and voltageripple ΔV_(R), for that particular signal level or band of signallevels. In each case the nominal headroom between a particular supplyvoltage and the corresponding output signal level or maximum outputsignal for a band of signal levels that all use the same supply voltage,would be relatively low, say of the order of 10% or less of themagnitude of the relevant supply voltage.

By contrast the present embodiments switch to the high voltage mode ofoperation based on a determination of when the DC-DC converter 102 willenter a current limited mode of operation and operates with asufficiently high voltage for a period before the higher signal level isincident on the amplifier such that the output capacitor storesadditional energy and can help support the load demand during allswitching phases of the DC-DC converter. The second mode of operationprovide excess headroom, significantly greater than what is required forconventional headroom. As discussed above in the second mode ofoperation of some embodiments of the nominal headroom may be at least20%, or at least 25% of the magnitude of the particular supply voltage.

In embodiments of the present disclosure the controller 301 maydetermine when the DC-DC converter 102 may enter a current limited modeof operation by determining when the signal level of the input signalexceeds a threshold V_(THR).

The threshold V_(THR) may be determined based on parameters of the DC-DCconverter 102 and the amplifier 104 and possibly other relevant systemparameters.

In one example the threshold V_(THR) may be determined or calculated asillustrated with respect to FIG. 5. The maximum output current that maybe supplied by the DC-DC converter 102, i.e. a power supply outputcurrent limit, may be determined based on the input current limitapplied to the DC-DC converter 102 and an efficiency value of the powersupply, i.e. the DC-DC converter 102. The current limit to be appliedwill be defined in use and can be a known value. The efficiency valuefor the DC-DC converter 102 may be a worst-case expected efficiencyvalue. This value will typically be known from design or simulation ofthe DC-DC converter but in some instances historic measured efficiencyvalues may be used.

Based on this maximum output current that can be supplied from the DC-DCconverter, before entering current limiting, and an efficiency value ofthe amplifier, an output current limit from the amplifier 104 may bedetermined. This would represent the maximum expected current that couldbe delivered from the amplifier 104 before the DC-DC converter 102 wouldenter current limiting. The efficiency value for the amplifier maylikewise be a known value based on design of the amplifier and/ormeasured values.

Based on the parameters of the load 105 being driven by the amplifier104 this maximum output current limit from amplifier 104 can beconverted to an indication of the maximum output amplitude, i.e. drivevoltage, that the amplifier 104 could drive without exceeding thecurrent limit. The load parameter may in particular be the loadimpedance R. In some instances the load inductance L may additionally oralternatively be used to determine the output amplitude limit. Theseimpedance and/or inductance values may be known for the relevant load,especially for an on-board loudspeaker, at the time of devicemanufacture. However in some instances these values could be assumedbased on the expected type of load or at least some measurement could bemade in use.

The current drawn by the load 105 for a certain amplitude driving signalmay also depend on the audio frequency and thus to determine anamplitude limit an indication of the minimum audio signal frequency mayadditional be taken into consideration.

Finally this output amplitude limit may be converted to an input signalamplitude limit based on the gain of the amplifier 104 (and possibly anygain adjustment in the signal path between the point at which thecontroller 301 taps the input signal and the amplifier 104).

In this way an input signal amplitude threshold V_(THR) may be defined,above which it is expected that the power demand would result in theDC-DC converter entering a current limited mode of operation.

Note that FIG. 5 illustrates the various steps in calculating athreshold V_(THR) in order to explain the principles that may be used indetermining an appropriate threshold. In practice it would not benecessary to perform these as individual steps in sequence and forinstance a single calculation could be performed with all relevantinputs and/or various of the inputs described above could be combined ina suitable way.

In some instances each of the inputs described may be known for aparticular implementation and may not vary in use. In such cases itwould be possible to determine a suitable threshold value to be storedor programmed to be available to controller 201. V_(THR) could forinstance be stored in memory or loaded into memory at start-up by systemsoftware.

In at least some embodiments however at least some of the inputs mayvary in use. For instance the input current limit to be applied to theDC-DC converter may be programmable and varied, in use, depending on thebattery state.

When the battery is relatively deeply discharged, current drawn from thebattery can cause a drop in battery voltage to a voltage so low that itresults in a system reset. The input current limit for the DC-DCconverter 102 may be set so as to avoid drawing too large a current forthis reason. However when the battery is fully charged the voltage dropis less of a problem as the voltage will not drop below the resetthreshold.

In some embodiments therefore the battery voltage may be monitored andthe input current limit controlled based on the battery voltage, withthe current limit being decreased as the battery discharges. This hasthe advantage of allowing the DC-DC converter 102 to draw more currentat high battery charge state and therefore the threshold V_(THR) levelcan be higher. A higher threshold V_(THR) means the amplifier circuitoperates in the lower voltage mode for a greater audio signal dynamicrange, increasing efficiency.

Thus in some embodiments the amplifier circuit may include a thresholdgenerator, e.g. a threshold calculation module as illustrated in FIG. 6.FIG. 6 shows an amplifier circuit 600 in which similar components areidentified by the same references numerals as used previously.

In FIG. 6 a threshold calculator 601 is provided to determine athreshold V_(THR) to be applied by controller 301 to determine whetherto operate with the high defined voltage V2 or a lower operatingvoltage, e.g. V1 or lower. The threshold calculator may comprise adedicated circuitry for determining the threshold value V_(THR) and/ormay comprise a software module, for instance running on a suitableprocessor.

The threshold calculator 601 may run at system power-on or reset toproduce V_(THR) and/or may operate continuously if the parameters usedto calculate the threshold vary in use.

The threshold calculator may receive at least some of the inputdescribed previously. The input current limit for the DC-Dc may beprogrammable and may be stored in memory and can be read by the V_(THR)Calculator. In some instances the input current limit may be adaptive asdescribed above.

Values for power supply (i.e. DC-DC converter) efficiency η and audioamplifier efficiency η may be stored in memory. The speaker resistance,inductance and minimum audio signal frequency may also be stored inmemory and may be programmable. In some instances a measure ofloudspeaker impedance and inductance and how it varies over time may bedetermined in use for some other reason, e.g. for speaker protection andin some instances the impedance and/or inductance value may be updatedin operation.

Likewise if the gain of the amplifier 104 is variable in use the gain ofthe amplifier may also be used to vary the threshold V_(THR).

The threshold value V_(THR) is provided to the controller 201. Thecontroller 201 also receives an indication of the input signal S_(IN) inorder to determine whether the level of input signal is above athreshold. In some embodiments the controller 201 receives the inputsignal S_(IN) itself and determines whether the signal level is abovethe threshold V_(THR).

The controller 301 may thus comprise a level detector 602, such as anenvelope detector, to determine an envelope level Env for the inputsignal S_(IN). The envelope detector 602 may, for example, peak detectthe input signal, with a relatively fast attack time to increases insignal level but with a decay time set with respect to the expectedsignal frequencies to maintain an envelope value.

A comparator 603 may be provided for comparing the determined signallevel with the threshold V_(THR). A mode controller 604 may beresponsive to the comparator 603 to generate a Mode control signal tocontrol an appropriate mode of operation of the DC-DC converter. In someinstances the Mode control signal be a suitable voltage reference signalfor indicating the desired output voltage to be generated by the DC-DCconverter 102.

To provide sufficient time to charge the output capacitor 103 to thedefined higher voltage, e.g. V2, before a high level part of the signalreaches the amplifier 104, without requiring the delay 302 to be undulylong, the controller should preferably respond quickly to any increasesin input signal level. As discussed the envelope detector 602 maytherefore have a fast attack time. In some embodiments the input signalreceived by the envelope detector 602 may be processed in some way, forinstance being filtered by a pre-emphasis filter to emphasise any risein signal level, as will be understood by one skilled in the art, so asto provide a fast response to increases in signal level.

To avoid unnecessary changes in mode between the defined high voltagemode applied when the DC-DC converter may be current limited and thelower voltage mode where current limiting is not expected, the envelopedetector may have a slower decay time and/or at least one of thecomparator or mode controller may apply some degree of hysteresis tochanging between modes. For example the voltage threshold V_(THR) may bethe threshold applied to switch into the defined high voltage mode ofoperation, but the high voltage mode of operation may be maintaineduntil the signal level drops below a lower threshold.

In some embodiments the controller 301 may not contain an explicit peakdetector and in some embodiments may not derive an actual envelopesignal Env, and the Mode control signal may be generated by other means,for instance a comparator coupled without peak detection to the inputsignal S_(IN), the comparator having hysteresis and/or a timeout, toeffectively provide a type of envelope detection and generate the Modecontrol signal.

Further in some embodiments the controller 301 may receive some otherindication regarding the amplitude or envelope of the input signal. Forinstance in some embodiments the signal level of the input signal may bedetermined by some upstream processing and provided to the controller orsome other signal indicative of the likely signal amplitude may beprovided.

Embodiments therefore use an indication of signal level to determinewhen the DC-DC converter 102 may enter experience current limiting andin particular determine when the input signal level exceeds a thresholdV_(THR) which is determined to correspond to likely current limited modeof operation. If the signal level is above the threshold the controller301 controls the DC-DC converter 102 in the second mode of operation,which is a defined high voltage mode, to produce an output voltage thatis higher than required for the peak voltage level of the amplifier. Thecontroller 301 operates in this second mode in order to pre-charge theoutput capacitor 103 to support a current demand during amplification ofthe high level part of the signal. If however the signal level remainsbelow the threshold the controller 301 may operate the DC-DC converterin a first mode with a lower output voltage.

In some embodiments further efficiency gains may be realised byoperating the DC-DC converter using Class-G or Class-H type techniquesin the first mode of operation, with the second mode of operation beinga different mode of operation.

As mentioned previous Class-G or Class-H type operation is a known typeof operation for amplifiers where the supply voltage V_(S) to theamplifier is adjusted based on an indication of signal level so as toprovide appropriate voltage headroom for the signal being amplified, orthe maximum signal level within a range, but to reduce the excessvoltage headroom.

When operating in the first mode of operation, i.e. when the DC-DCconverter is not operating in a current limited mode of operation, theoutput voltage of the converter may thus be controlled based on anindication of the signal level to control the supply voltage to trackthe signal level, either substantially continuously between a minimumvoltage and the voltage threshold V_(THR) in a Class-H implementation,or to one of a plurality of defined voltage levels for defined bands ofsignal level for a Class-G type implementation.

The envelope detector 602 of controller 301 may thus provide an envelopevalue that can be used by mode controller 604 to implement Class-Htracking or which may be used by comparator 603 to compare against aplurality of different voltage thresholds to determine an appropriatesupply voltage to be supplied by the DC-DC converter 102. The first modeof operation may therefore be seen as having various sub-modes ofoperation or as a mode with a variable voltage based on voltageheadroom.

In some embodiments the DC-DC converter 102 may have a minimum outputvoltage that can be achieved. Thus in some embodiments the modecontroller 604 may control the DC-DC converter 102 in the first sub-modeof operation for input signal levels which are lower than the currentlimiting threshold V_(THR) and may vary the output voltage of the DC-DCconverter between the minimum voltage V_(MIN) and a maximum voltage V1for the first mode of operation. The voltage V1 will correspond to avoltage which provides sufficient headroom for amplifying signals whichare just below the threshold V_(THR) for current limiting. Thus, asdescribed previously, V1 may be set to provide a nominal headroom thatis sufficient for the required operating headroom H_(O), i.e. theamplifier headroom H_(A) and voltage ripple ΔV_(R).

For a boost converter, i.e. a DC-DC converter with a step-up topology,the minimum output voltage may correspond to a step-up factor of 1, i.e.the minimum output voltage may be equal to the input voltage. In such acase rather than operate the DC-DC converter 102 to effectively providethe battery voltage as an output, with associated power consumption dueto operation of the DC-DC converter 102, in some embodiments thecontroller 301 may also be operable to control the DC-DC converter 102to operate in a bypass mode. In the bypass mode the switching componentsof the DC-DC converter 102 may be disabled, or at least the switchingfrequency significantly reduced to reduce power consumption, with thebattery voltage being supplied directly to the output. The DC-DCconverter 102 may therefore comprise part of a voltage regulator 605,wherein in the bypass mode the voltage regulator may disable the DC-DCconverter 102 and deliver the battery voltage as the output voltage ofthe voltage regulator.

Thus if the input signal level is such that the battery voltage itselfwould provide sufficient conventional voltage headroom for the signal tobe amplified, taking into account the gain applied by the amplifier 104and any downstream processing between the controller 301 and theamplifier 104, the mode controller 603 may control the DC-DC converter102 in a bypass mode of operation. If the signal level increases so thatthe battery voltage is no longer sufficient, but the signal levelremains below the current limiting threshold V_(THR) the DC-DC converter102 may be controlled with an appropriate voltage reference to providesufficient conventional voltage headroom for the signal being amplified.If the signal level exceeds the current limiting threshold V_(THR) theDC-DC converter 102 is switched to the second mode of operation and theoutput voltage of the DC-DC converter 102 is increased to the highvoltage V2 and thus provide excess voltage headroom.

In some embodiments however the DC-DC converter may be a buck-boostconverter capable of both step-up and step-down operation. In which casethe DC-DC converter 102 may always be operated with the output voltagebeing controlled in the first mode between the minimum output voltage,which may be lower than the battery voltage V_(BATT), and the maximumoutput voltage V1 for the first mode of operation.

The maximum voltage output by the DC-DC converter 102 in use will be thehigh-defined voltage V2 used in the second mode of operation. Thisvoltage may be programmable to account for different manufacturers usingoutput capacitors of different sizes. The controller 301 may thus beresponse to an indication V_(MAX) of the high voltage to be used in thesecond mode. The controller 301 may also be provided with an indicationV_(MIN) of the minimum voltage that can be achieved by the DC-DCconverter 102 and also an indication of the Gain applied by theamplifier 104.

As mentioned above the delay 302 may be set so as to allow time for theoutput capacitor to charge to the defined high voltage, i.e. V_(MAX),before the high level part of the signal reaches the amplifier 102. Theamount of the delay may be determined by the largest change in supplyvoltage. For the step-up topology this may be the time to enable theDC-DC converter 102 to charge the output capacitor from an initialvoltage of V_(BATT) up to the defined high voltage V_(MAX). For abuck-boost converter the required voltage change could be from V_(MIN)to V_(MAX). The voltage slew rate of the supply voltage is known fromthe DC-DC converter design and depends on at least some of the topology,switching frequency, inductor ripple current, output capacitor and load.

In some embodiments the delay may be programmable to allow differentchoice of DC-DC converter components, e.g. inductor and outputcapacitor, and also switching frequency.

The delay 302 may also include additional delay for digital to analogueconversion and/or pulse width modulation as part of the audio amplifierimplementation. This portion of the delay is dependent on amplifiertopology, is known from design and has a fixed duration.

FIG. 7 illustrates voltage waveforms similar to those illustrated inFIGS. 2 and 4 to show one example of operation according to theembodiment of FIG. 6. FIG. 7 illustrates the same amplified signal 201as shown in FIGS. 2 and 4.

In this example initially the signal level is at a level where thebattery voltage V_(BATT) would provide sufficient voltage headroom forthe amplifier 104. Thus, as described above the controller 301, may, insome instances, operate the DC-DC converter 102 in a bypass mode ofoperation and the battery voltage may be provided directly as the outputvoltage of the voltage regulator. This may, for instance, be the casewhere the DC-DC converter 102 is a boost converter.

At a time ta the level of the signal being amplified has increased suchthat the battery voltage V_(BATT) is no longer sufficient. Thecontroller 201 thus enables the DC-DC converter 102 and increases itsoutput voltage to a level sufficient for this increased signal level. Itwill be understood that the controller 301 will be aware of this signallevel increase at a time before the increase reaches the amplifier by anamount equal to the delay time D. The amount of this delay is set withregard to the maximum time taken to slew to the maximum output voltageV2 in the second mode of operation and thus may be a greater delay thanis required for Class-H tracking. The controller can thus determine theappropriate time to implement the relevant increase or decrease inoutput voltage. Between time ta and t0 the output voltage of the DC-DCconverter may be controlled to track the amplitude of the signal beingamplified with appropriate headroom. Purely for the purposes ofillustration FIG. 7 shows that with a buck-boost converter the supplyvoltage may also be controlled to be lower than the battery voltage whenappropriate.

At time t0, in a similar manner as described above, it is detected thatthe input signal level exceeds the current limiting threshold V_(THR).Thus the output voltage of the DC-DC converter 102 is increased to thedefined high voltage V2 to charge the output capacitor 103 to thisvoltage. This additional energy stored in the output capacitor 102supports the load current demand during this period of high signal leveluntil time t2, where the signal 201 being amplified has again dropped toa low level and thus the DC-DC converter 102 may be switched back to thefirst mode of operation and may resume Class-H operation.

By way of contrast for the purposes of explanation FIG. 7 alsoillustrates, by dashed line 701, the operation of the voltage supplythat would be implemented if current limiting were not to occur. In sucha case it would be expected that the supply voltage V_(S) would trackthe signal level in the same way as illustrated up until time t0. Inthis case the high level part of the signal, which is output betweentime t1 and t2, would still be detected in advance of it being amplifiedbut the amount of look ahead required may be less. The supply voltage isthus increased before the high level part of the signal is amplified att1 but the supply voltage only tracks the signal level with conventionalheadroom. The supply voltage during the high level part of the signalbetween t1 and t2 is thus much lower than the voltage V2 used to provideexcess headroom when current limiting is an issue. As such less time isrequired to charge the output capacitor 103 to the required outputvoltage and the supply voltage can be maintained on a cycle-by-cyclebasis of the DC-DC converter 102. FIG. 7 illustrates that the highvoltage V2 used in the second mode of operation thus has significantexcess headroom compared to the standard headroom for class G/Htracking.

Embodiments of the present disclosure thus provide amplifier arrangementthat can meet audio power requirements whilst offering good efficiencyand without requiring unduly large capacitances. Embodiments allow thetechniques of Class-G and/or class-H type amplifiers to be used with aDC-DC converter that may experience current limiting in use due to adefine battery current limit. A defined high voltage mode may be enteredwhen it is determined that the input signal level exceeds a threshold atwhich current limiting is expected. In some embodiments a thresholdcalculator may calculate an appropriate threshold based on one or morevariable parameters, such as an input current limit for the DC-DCconverter.

The skilled person will recognise that some aspects of theabove-described apparatus and methods, for example the discovery andconfiguration methods may be embodied as processor control code, forexample on a non-volatile carrier medium such as a disk, CD- or DVD-ROM,programmed memory such as read only memory (Firmware), or on a datacarrier such as an optical or electrical signal carrier. For manyapplications embodiments of the invention will be implemented on a DSP(Digital Signal Processor), ASIC (Application Specific IntegratedCircuit) or FPGA (Field Programmable Gate Array). Thus the code maycomprise conventional program code or microcode or, for example code forsetting up or controlling an ASIC or FPGA. The code may also comprisecode for dynamically configuring re-configurable apparatus such asre-programmable logic gate arrays. Similarly the code may comprise codefor a hardware description language such as Verilog™ or VHDL (Very highspeed integrated circuit Hardware Description Language). As the skilledperson will appreciate, the code may be distributed between a pluralityof coupled components in communication with one another. Whereappropriate, the embodiments may also be implemented using code runningon a field-(re)programmable analogue array or similar device in order toconfigure analogue hardware.

Note that as used herein the term module shall be used to refer to afunctional unit or block which may be implemented at least partly bydedicated hardware components such as custom defined circuitry and/or atleast partly be implemented by one or more software processors orappropriate code running on a suitable general purpose processor or thelike. A module may itself comprise other modules or functional units. Amodule may be provided by multiple components or sub-modules which neednot be co-located and could be provided on different integrated circuitsand/or running on different processors.

Embodiments may be implemented in a host device, especially a portableand/or battery powered host device such as a mobile telephone, an audioplayer, a video player, a PDA, a mobile computing platform such as alaptop computer or tablet and/or a games device for example. The hostdevice may comprise one or more audio output transducers, e.g.loudspeakers to be driven by an amplifier arrangement as described.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. The word “comprising” does not excludethe presence of elements or steps other than those listed in a claim,“a” or “an” does not exclude a plurality, and a single feature or otherunit may fulfil the functions of several units recited in the claims.Any reference numerals or labels in the claims shall not be construed soas to limit their scope.

The invention claimed is:
 1. An amplifier circuit for amplifying anaudio signal, comprising: a voltage regulator having an input node forreceiving an input voltage and an output node for outputting an outputvoltage, the voltage regulator comprising an output capacitor coupled tothe output node; an amplifier connected to receive the output voltagefrom said output node of the voltage regulator as a supply voltage; thevoltage regulator being operable in a voltage-control mode to maintainthe output voltage at a nominal output voltage and in current-controlmode to apply current limiting so as to limit an input current drawn bythe voltage regulator via the input node so as not to exceed a definedinput current limit; and a controller for defining the nominal outputvoltage; wherein the controller is operable in a first mode to definethe nominal output voltage so as not to exceed a first voltage magnitudeand is operable in a second mode to define the nominal output voltage tobe equal to a second voltage magnitude, the second voltage magnitudebeing greater than the first voltage magnitude; wherein the controlleris configured to monitor an indication of the level of the audio signalfor a high-amplitude part of the audio signal that could result in thevoltage regulator operating in the current-control mode to apply currentlimiting; and wherein the controller is configured to operate in saidfirst mode until such a high-amplitude part of the audio signal isdetected and, on such detection to operate in the second mode ofoperation until such a high-amplitude part of the audio signal has beenamplified; wherein the second voltage magnitude is greater than requiredfor voltage headroom for amplifying the high-amplitude part of the audiosignal so as to allow for a voltage droop of the output voltage over aplurality of switching cycles of the voltage regulator when operating inthe current-control mode.
 2. An amplifier circuit as claimed in claim 1wherein, on detection of a high-amplitude part of the audio signal, thecontroller is configured to change to the second mode of operation suchthat the output capacitor is substantially charged to the second voltagemagnitude before the high-amplitude part of the audio signal isamplified by the amplifier.
 3. An amplifier circuit as claimed in claim1 wherein the controller is configured to detect a high-amplitude partof the audio signal by determining when the indication of the level ofthe audio signal exceeds a first threshold corresponding to a signallevel that would be expected to lead to the voltage regulator operatingin the current-control mode to applying current limiting.
 4. Anamplifier circuit as claimed in claim 3 comprising a threshold generatorfor determining said first threshold.
 5. An amplifier circuit as claimedin claim 4 wherein the threshold generator is configured to determinesaid first threshold based on said defined input current limit.
 6. Anamplifier circuit as claimed in claim 5 wherein the defined inputcurrent limit is configurable and is configured to vary with a batteryvoltage at said input node.
 7. An amplifier circuit as claimed in claim3 wherein the threshold generator is configured to determine said firstthreshold based on at least one of: voltage regulator efficiency;amplifier efficiency; audio load impedance; audio load inductance; aminimum audio frequency; and amplifier gain.
 8. An amplifier circuit asclaimed in claim 1 the second voltage magnitude is greater than requiredfor voltage headroom for amplifying the high-amplitude part of the audiosignal so as to allow for a voltage droop of the supply voltage of atleast 15%.
 9. An amplifier circuit as claimed in claim 1 wherein thecontroller is configured such that in said first mode of operation thenominal output voltage provides a nominal headroom of 10% or less of themagnitude of the nominal output voltage, wherein the nominal headroom isthe voltage difference between the nominal output voltage and themaximum output voltage of the amplifier.
 10. An amplifier circuit asclaimed in claim 1 wherein the controller is configured such that insaid second mode of operation the nominal output voltage provides anominal headroom of at least 25% of the magnitude of the nominal outputvoltage, wherein the nominal headroom is the voltage difference betweenthe nominal output voltage and the maximum output voltage of theamplifier.
 11. An amplifier circuit as claimed in claim 1 comprising adelay in a signal path for the audio signal upstream of the amplifierwherein said delay has a delay duration which is at least equal to thetime that would be required for the output capacitor to be charged tofrom a lowest expected voltage level in the first mode of operation tothe second voltage magnitude.
 12. An amplifier circuit as claimed inclaim 11 wherein said delay comprises a buffer of configurable size. 13.An amplifier circuit as claimed in claim 1 wherein in the first mode ofoperation the controller is configured to vary the nominal outputvoltage of the voltage regulator based on the indication of the level ofthe audio signal.
 14. An amplifier circuit as claimed in claim 1wherein, in the first mode of operation, the controller is operable tocontrol the voltage regulator in a pass-through mode to pass the inputvoltage directly to the output node as the supply voltage if theindication of the level of the audio signal is below a second threshold.15. An amplifier circuit as claimed in claim 1 wherein the voltageregulator comprises a DC-DC converter and the controller is configuredto vary a voltage reference signal supplied to the DC-DC converter todefine the nominal output voltage.
 16. An amplifier circuit as claimedin claim 1 comprising a battery, wherein the input node of the voltageregulator is coupled to receive the input voltage from said battery. 17.An amplifier circuit as claimed in claim 1 wherein the defined inputcurrent limit corresponds to a battery current limit.
 18. An electronicdevice comprising an amplifier circuit as claimed in claim 1 wherein theelectronic device is at least one of: a portable device; a batteryoperated device; a communication device; a mobile or cellular telephonedevice; a smartphone; a computing device; a laptop, notebook or tabletcomputing device; a gaming device; a personal media player; a wearabledevice.
 19. An amplifier circuit for amplifying an audio signal,comprising: an amplifier; a voltage regulator having an input node forreceiving an input voltage and an output node for outputting a supplyvoltage to said amplifier, the voltage regulator comprising an outputcapacitor coupled to the output node; the voltage regulator comprising avoltage control loop for controlling the voltage at the output node to anominal output voltage and a current control loop for, when activated,limiting an input current drawn by the voltage regulator via the inputnode so as not to exceed a defined input current limit, wherein thevoltage regulator operates according to the voltage control loop unlessthe current control loop is activated; and a controller for defining thenominal output voltage; wherein the controller is operable in a firstmode to define the nominal output voltage so as not to exceed a firstvoltage magnitude and is operable in a second mode to define the nominaloutput voltage to be equal to a second voltage magnitude, the secondvoltage magnitude being greater than the first voltage magnitude;wherein the controller is configured to monitor an indication of thelevel of the audio signal for a high-amplitude part of the audio signalthat could result in the current control loop of the voltage regulatorbeing activated; and wherein the controller is configured to operate insaid first mode until such a high-amplitude part of the audio signal isdetected and, on such detection to operate in the second mode ofoperation until such a high-amplitude part of the audio signal has beenamplified; wherein the second voltage magnitude is greater than requiredfor voltage headroom for amplifying the high-amplitude part of the audiosignal so as to allow for a voltage droop of the supply voltage over aplurality of switching cycles of the current control loop dominating thevoltage control loop.
 20. An amplifier circuit for amplifying an audiosignal, comprising: an amplifier; a voltage regulator having an inputnode for receiving an input voltage and an output node for outputting asupply voltage to said amplifier, the voltage regulator comprising anoutput capacitor coupled to the output node; the voltage regulator beingoperable to apply current limiting so as to limit an input current drawnby the voltage regulator via the input node so as not to exceed adefined input current limit; a controller for defining a nominal outputvoltage generated by the voltage regulator; wherein the controller isoperable to compare an indication of the level of the audio signal to afirst threshold, wherein the first threshold is indicative of the inputsignal that, if amplified by the amplifier, would be expected to lead tothe voltage regulator experiencing current limiting; and in the eventthat the indication of the level of the audio signal exceeds said firstthreshold to increase the magnitude of the nominal output voltage of thevoltage regulator from a first operating range to a second highervoltage level.